Liquid crystal display devices using nematic liquid crystal display elements have been widely used as numerical-value-segment type liquid crystal display devices such as watches and desktop calculators. In recent years, such a liquid crystal display device is used as a display device of word processors, computers, navigation systems, etc.
In general, such a liquid crystal display device includes light transmitting substrates on which electrode lines, etc. are formed for switching on/off pixels. For example, in an active matrix liquid crystal display device, active elements such as thin film transistors (TFT) are formed on the substrate. The active elements function as a switching means for selectively driving pixel electrodes through which a voltage is applied to a liquid crystal layer. Additionally, in a liquid crystal display device capable of providing a color display, color filter layers, for example, red, green and blue filters, are placed on the substrate.
A liquid crystal display element of the above-mentioned liquid crystal display device is fabricated by sealing the liquid crystal layer in the space between a pair of light transmitting substrates. A transparent electrode layer and an alignment film are formed on each of the facing surfaces of the light transmitting substrates. Moreover, a pair of polarizers (polarizing plates) are disposed on both sides of the liquid crystal display element, respectively. The liquid crystal display element and the polarizers form a liquid crystal display device.
Twisted nematic liquid crystals are often used in a liquid crystal display device using the above-mentioned nematic liquid crystal display element. The alignment films formed on the facing surfaces of the pair of light transmitting substrates are rubbed in intersecting directions, respectively. Liquid crystal molecules which are sealed in the space between the light transmitting substrates are aligned according to the rubbing directions. The liquid crystal molecules are twisted in a spiral form from one of the substrates toward the other through the liquid crystal layer.
The display method of such a liquid crystal display device is classified into the following two known modes according to the twist angle of the nematic liquid crystals.
(a) The active-driving-type twisted nematic liquid crystal display mode (hereinafter referred to as the TN mode) in which nematic liquid crystal molecules are aligned in a 90.degree.-twisted state.
(b) The multiplex-driving-type super-twisted nematic liquid crystal display mode (hereinafter referred to as the STN mode) in which nematic liquid crystal molecules are aligned in a twisted state with a twist angle of not smaller than 90.degree..
The latter display mode (b), namely the STN mode, suffers from a peculiar coloring phenomenon. Therefore, when providing a black-and-white display in the STN mode, the use of an optical compensation plate is considered to be effective. The STN modes using an optical compensation plate are mainly classified into the following two modes.
(b-1) The double layered super-twisted nematic liquid crystal display mode using a display-use liquid crystal cell, and a liquid crystal cell which is oriented with a twist angle in a direction opposite to that of the display-use liquid crystal cell.
(b-2) The film-stacked liquid crystal display mode.
If these two modes are compared to each other, the film-stacked liquid crystal display mode is found more advantageous in terms of the weight and cost, i.e., the film-stacked liquid crystal display mode allows a lighter weight and a lower cost.
Regarding the former display mode (a), namely the TN mode, two operation modes are selected by arranging the polarization directions of the polarizers to be orthogonal or parallel to each other. The TN mode is mainly classified into the following two types according to the operation modes.
(a-1) The normally black mode in which a pair of polarizers are disposed so that their polarizing directions are parallel to each other, and a black display is provided in a state (OFF state) in which a voltage is not applied to the liquid crystal layer.
(a-2) The normally white mode in which a pair of polarizers are disposed so that their polarizing directions cross each other at a right angle, and a white display is provided in the OFF state.
If these two modes are compared to each other, the latter normally white mode is found more advantages in terms of the display contrast, color reproducibility, and viewing angle dependence of display.
However, the TN mode has a problem that the contrast of a displayed image varies according to the viewing direction or angle of a viewer, i.e., high viewing angle dependence. Such a problem is caused by that the liquid crystal molecules possess index anisotropy .DELTA.n, and aligned in an inclined state with respect to the upper and lower substrates.
For example, FIG. 7 shows a depiction of the cross section of a TN liquid crystal display element 31 when an half-tone display voltage is applied to the TN liquid crystal display element 31. It is seen from FIG. 7 that liquid crystal molecules 32 move into a slightly raised state.
In this case, in the liquid crystal display element 31, linearly polarized light 35 which passes in a normal direction to the surfaces of substrates 33.cndot.34 and linearly polarized light 36.cndot.37 which passes in an inclined direction with respect to the normal direction intersect the liquid crystal molecules 32 at different angles. Since the liquid crystal molecules 32 possess index anisotropy .DELTA.n, when the linearly polarized light 35.cndot.36.cndot.37 traveling in the above-mentioned directions passes through the liquid crystal molecules 32, an ordinary ray and an extraordinary ray are produced. The light transmitted through the liquid crystal display element 31 is converted into elliptically polarized light because of the phase difference between the ordinary ray and extraordinary ray. This conversion is the source of the viewing angle dependence.
Moreover, in an actual liquid crystal layer, the tilt angle of the liquid crystal molecules 32 differs between the vicinity of the intermediate position of the substrates 33.cndot.34 and the vicinity of the substrates 33.cndot.34. Furthermore, the liquid crystal molecules 32 are twisted 90.degree. about an axis that is the normal direction to the substrates 33.cndot.34. Thus, the linearly polarized light 35.cndot.36.cndot.37 receives various birefringence effects according to the incident direction and angle, and shows complex viewing angle dependence.
The viewing angle dependence will be explained in detail below. When the viewing direction is inclined certain degrees from the normal direction toward a lower direction of the screen, i.e., a positive viewing direction, a phenomenon in which the displayed image is colored (hereinafter referred to the "coloring phenomenon) or a phenomenon in which black and white are switched over (hereinafter referred to as the "inverting phenomenon") is observed. On the other hand, when the viewing direction is inclined toward an upper direction of the screen, i.e., a negative viewing direction, the contrast is abruptly lowered.
In addition, the above-mentioned liquid crystal display device has a problem that the viewing angle is narrowed with an increase in the area of the screen. For example, when a large-area liquid crystal display screen is viewed from a position in front of the screen at a close distance, the color displayed in the upper part of the screen and the color in the lower part are sometimes seen in different colors due to the influence of the viewing angle dependence. This problem is caused by that the apparent angle in viewing the entire screen increases, and the same result as viewing the liquid crystal display screen from a further inclined direction is produced.
In order to improve the viewing angle dependence, it has been proposed to insert a phase-difference plate (phase-difference film) as an optical element having optical anisotropy between the liquid crystal display element and one of the polarizers (see, for example, Japanese laid-open patent applications, No. 000600/1980 (Tokukaisho 55-000600), and No. 097318/1981 (Tokukaisho 56-097318).
This proposed method improves the viewing angle dependence as follows. Linearly polarized light is converted into elliptically polarized light when passing through liquid crystal molecules having index anisotropy. The resulting light is caused to pass through the phase-difference plate provided on one or both sides of a liquid crystal layer having index anisotropy so as to compensate for the phase difference between an ordinary ray and an extraordinary ray that varies according to the viewing angle and reconvert the light into linearly polarized light. In this method, therefore, it is necessary to set up not only the characteristics of the phase-difference plate, but also the characteristics of the liquid crystal layer, i.e., the liquid crystal display element.
Then, as a method for further improving the viewing angle dependence, Japanese laid-open patent application, No. 313159/1993 (Tokukaihei 5-313159) proposes a liquid crystal display device using a phase-difference plate in which the direction of one of the principal refractive indices of an index ellipsoid is parallel to a normal direction to the surface of the phase-difference plate, and a liquid crystal element in which .DELTA.n.multidot.d is the product of .DELTA.n (=n.sub.x -n.sub.y) and the cell thickness d (liquid crystal layer thickness) of the phase difference plate is within a range of from 200 nm and 500 nm, wherein the phase-difference plate is disposed between the liquid crystal display element and the polarizer.
In this liquid crystal display device, not only the characteristics of the phase-difference plate and the liquid crystal display element are set, but also the rubbing direction of an alignment film constituting the liquid crystal display element, the lagging axis direction of the phase-difference plate and the transmitting axis direction of the polarizer are made parallel to each other. This structure allows a further improvement of the viewing angle dependence of the liquid crystal display device.
Furthermore, a liquid crystal display device using a phase-difference plate in which the principal refractive index direction of the index ellipsoid is inclined with respect to the normal direction to the surface of the phase-difference plate has been proposed (see U.S. Pat. No. 5,506,706). This liquid crystal display device uses the following two kinds of phase-difference plates.
In one of the phase-difference plates, the direction of the minimum principal refractive index among three principal refractive indices of the index ellipsoid is parallel -to the surface of the phase-difference plate. The direction of one of the remaining two principal refractive indices is inclined an angle of .theta. with respect to the surface of the phase-difference plate. The direction-of the other principal refractive index is inclined an angle of .theta. with respect to the normal direction to the surface of the phase-difference, plate. In this phase-difference plate, the value of .theta. satisfies 20.degree..ltoreq..theta..ltoreq.70.degree..
In the other phase-difference plate, there is no index anisotropy within the surface of the phase-difference plate. The principal refractive index nb in the normal direction to the surface of the phase-difference plate, and the principal refractive indices na and nc parallel to the surface of the phase-difference plate satisfy the relationship na=nc&gt;nb. Namely, this phase-difference plate has an optically negative uniaxial property. Moreover, the index ellipsoid is inclined by turning the direction of the principal refractive index nb clockwise or counterclockwise about an axis that is the direction of one of the principal refractive indices na and nc parallel to the surface of the phase-difference plate from a state parallel to the normal direction to the surface of the phase-difference plate to an inclined state.
As the former phase-difference plate, it is possible to use either an optically uniaxial plate or an optically biaxial plate. As the latter phase-difference plate, it is possible to use not only one phase-difference plate, but also a combination of two phase-difference plates wherein the inclined directions of the respective principal refractive indices nb with respect to the normal direction to the surfaces of the phase-difference plates form 90.degree. together.
In a liquid crystal display device constructed by placing at least one piece of such a phase-difference plate between the liquid crystal display element and the polarizer, the viewing angle dependence is improved to a certain level.
However, in the method disclosed by the above-mentioned document, Japanese laid-open patent application No. 313159/1993 (Tokukaihei 5-313159) wherein the phase-difference plate in which the direction of one of the principal refractive indices of the index ellipsoid is parallel to the normal direction to the surface of the phase-difference plate is placed between the polarizer and the liquid crystal display element including a liquid crystal layer whose retardation .DELTA.n.multidot.d is set within a range of from 200 nm to 500 nm, the viewing angle dependence can be improved only in a certain direction of the screen, i.e., cannot be improved in every direction. Namely, this method faces its limitation.
Furthermore, in the method of using the phase-difference plate as disclosed in the above-mentioned document U.S. Pat. No. 5,506,706, the conditions are set so that the index ellipsoid is inclined. A liquid crystal display element which is formed by a liquid crystal material with index anisotropy .DELTA.n of 0.08 and a liquid crystal layer with a thickness d of 4.5 .mu.m, i.e., a liquid crystal element in which the value of retardation .DELTA.n.multidot.d of the liquid crystal layer is 360 nm, is used as one embodiment of the liquid crystal display element. However, this document does not provide more information about the liquid crystal display element and the polarizer between which the phase-difference plate is to be placed.
In the method of improving the viewing angle dependence by placing the phase-difference plate between the liquid crystal display element and the polarizer, it is necessary to set up not only the characteristics of the phase-difference plate, but also the characteristics of the liquid crystal display element. In this method, therefore, it is not clear that which range of .DELTA.n.multidot.d of the liquid crystal display element is to be used together with the phase-difference plate in order to achieve most effective compensation for a variation in the phase difference. Thus, the use of the above-mentioned phase-difference plate for a reduction of the viewing angle dependence of the liquid crystal display device has not produced satisfactory results.